Known recording mediums for recording and reproducing information signals, such as that for audio or video, include disc-shaped optical recording mediums and disc-shaped magnetic recording mediums.
Among these recording mediums, there are optical discs, on which information signals are written as micro-irregularities, such as by pits and grooves, a phase-change optical disc, a magneto-optical disc, exploiting photomagnetic effects of the recording film, and a hard disc for magnetically written signals.
For forming a recording layer on an optical recording medium, among these recording mediums, having micro-irregularities, such as phase pits or pre-grooves, in association with information signals, such as data information or tracking servo signals, injection molding of a substrate of plastics material is routinely employed. Specifically, a disc-shaped substrate is formed using an injection molding device, a metal mold and a stamper, and information signals are transcribed at this time from the stamper.
To readout and record information to such optical discs usually a laser beam with wavelength λ is focused through a simple objective lens of given numerical aperture NA onto the recording layer through a light transmitting layer of thickness d>>λ and at a working distance WD>> between the objective lens and the surface of the light transmitting layer. The spot diameter D of the focused laser beam is given hereby as D=/NA. Commercially available discs like Compact Discs (CD, =780 nm, NA=0.45, d=1.2 mm), Digital Versatile Discs (DVD, =650 nm, NA=0.60, d=0.6 mm), High Definition Digital Versatile Discs (HD-DVD, =405 nm, NA=0.65, d=0.6 mm) or Blu-ray Discs (BD, =405 nm, NA=0.85, d=0.1 mm) are using this Far Field Optic principle. By reducing  and increasing NA the spot diameter D can be reduced and therefore the data density can be increased.
However in such Far Field Optics (d and WD>>) the NA of the objective lens is limited to values <1.0. To further increase the data density, the NA has to become larger than 1.0 which can be realized by Near Field Optics (NFR). One implementation of NFR can be by utilizing a so called Solid Immersion Lens (SIL) (see, e.g., S. M. Mansfield, W. R. Studenmund, G. S. Kino, and K. Osato, “High-numerical-aperture lens system for an optical storage head,” Opt. Lett. 18, 305 ff (1993), the entire contents of which are incorporated herein by reference). For example, in a lens system composed of a Far Field lens of NA<1.0 and a hemispherical lens made of a material with index of refraction nSIL, the effective numerical aperture NAeff is given by NA·nSIL which will exceed 1.0 if nSIL is large enough. Another implementation could be by a small aperture of diameter DAp<<, which can be realized either by fiber optics with a very narrow end aperture (see, e.g., H. Brückl, Physik in unserer Zeit, 28, Jahrgang 1997 Nr. 2, p 67 if, the entire contents of which are incorporated herein by reference) or by an optically non linear responding thin masking layer (so called Super Resolution Enhanced Near Field Structure, see, e.g., J. Tominaga, et al., Applied Physics Letters, Vol 73 (15) 1998 pp. 2078-2080, the entire contents of which are incorporated herein by reference).
NFR utilizes the electromagnetic field at a WD<< between the surface of the lens system or aperture and the surface of the disk or the recording layer. For example, in K. Saito et al., Technical Digest ISOM 2001, p 244 ff, it was shown that at a working distance W<<405 nm, sufficient light of the evanescent wave of a SIL can be coupled into the disc, so that NAeff of that SIL can be increased above the Far Field limit of 1.0. Also it was shown that the accuracy of WD has to be controlled to a level of a few nm in order to get a stable reproducing signal. This can be understood as the intensity of the evanescent wave decays exponentially with the distance from the lens surface. To establish such a control mechanism an active feed back servo loop was proposed and introduced by T. Ishimoto et al., Technical Digest ISOM/ODS 2002, WC3, p 287 ff, the entire contents of which are incorporated herein by reference. This servo loop is able to compensate also fluctuations of WD coming from modal oscillations of the rotating disc (J. I. Lee et al., Technical Digest ODS 2006 MC4, p 43 ff, the entire contents of which are incorporated herein by reference). However due to band width limitations of the servo loop such compensation works only well at lower disk rotation speeds and for low frequency modal oscillations with modal frequencies <800 Hz. Therefore a limitation in data transfer rates exists due to the amplitudes of the high frequency modal oscillations of an e.g. 1.1 mm thick massive polycarbonate disc with 120 mm diameter. The substrate disclosed therein does not fulfill the requirements of the substrate of the present invention. In order to improve the gap servo control operation also at high disc rotation speed especially the high frequency modal oscillation behavior of the disc has to be improved.
The modal oscillation is characterized by its modal frequency fn which is related to the geometry of the disc and the ration of Young's modulus E and mass density ρ according to fn proportional to (E/ρ)0.5 (see also equation 1). The quality factor Q (see equation 2) is related to tan δ via Q=3/tan δ. In that sense Q can be used as a measure of the damping like tan δ. Low Q means high damping as tan δ is high. In general E and Q shows a distinct dependency on frequency f.
U.S. Pat. No. 6,908,655 B2, the entire contents of which are incorporated herein by reference, focuses on influencing the low frequency (first) modal oscillations, that occur on a typical 1.1 mm thick massive polycarbonate disc of 120 mm diameter around 140 Hz and is also related to a far field optical pick up head.
WO 00/48172, the entire contents of which are incorporated herein by reference, is focussed on the first modal frequency (<300 Hz) behaviour of a disc and it is said that the first modal frequency should be preferably located outside the rotation operating range of the disc. With respect to the behavior of the high frequency modal oscillations (>=2000 Hz) no solution is disclosed. Comparative example 3 which is presented in the experimental part of the present application, based on example 2 of WO 00/48172, shows that solutions that fulfill the low frequency requirements with respect to damping do not meet the high frequency requirements of the present invention.
WO 2003/005354A1, the entire contents of which are incorporated herein by reference, describes special copolycarbonates to achieve improved damping of the discs. This disclosure differs from the present invention with respect to either the chemical structure of the polymer or describes the low (first) modal frequency requirements with respect to damping but does not describe the high frequency requirements of the present invention.
Further solutions to achieve improved damping at low frequencies (1 Hz-16 Hz), which however are not sufficient for the high frequency modal oscillation requirements of the present invention were published in U.S. Pat. No. 6,391,418 B1, EP 1 158 024 A1 and US 2004/0265605 A1, the entire contents of each of which are incorporated herein by reference. U.S. Pat. No. 6,391,418 B1 describes a substrate for information recording media made of a polycarbonate composition comprising polycarbonate of a viscosity-average molecular weight of 10.000 to 40.000 and on biphenyl, a terphenyl compound or a mixture thereof. EP 1 158 024 A1 describes a vibration-damping thermoplastic resin composition comprising a) 50-90 wt. % of an amorphous thermoplastic resin having a loss tan δ of 0.01 to 0.04 and a deflection temperature under load of not lower than 120° C. and b) 50 to 10 wt. % of a methyl methacrylate resin wherein the article molded therefrom has certain physical properties. US 2004/0265605 A1 describes a vibration damping storage medium for data comprising a substrate, a physical portion of which comprises at least one polyimide and at least one data layer on the substrate. It is related to the first modal (low frequency) oscillation.
Another important feature for NFR is, the ability to couple light from the evanescent field through the WD<< into the surface of the recording medium to fully utilize the NAeff of the SIL for reducing D to /NAeff. For that the real part n of the index of refraction of the uppermost light transmitting layer of the recording medium has to be larger than NAeff. Such a layer can be realized by a high refractive index layer (HRI coating) which according to the invention may form the uppermost layer of recording medium and allows the coupling of light in the evanescent field into the recording medium. The HRI coating may also be used as a spacer layer between two or more reproducing layers or recording layers. To increase the storage density at least by a factor of 2, in comparison to the best respective Far Field optic (NA<1.0) NAeff should be at least >1.41 and therefore the real part n of the index of refraction of the HRI layer should be at least >1.41. Prior art focused on Far Field optics did not have to account for that.
U.S. Pat. No. 6,875,489 B2 or EP 1,518,880 A, the entire contents of each of which are incorporated herein by reference 1 are focusing on light transmitting layers with thicknesses d>3 μm, as these embodiments are related to Far Field optical discs like BD. As with NFR the effective NAeff is larger than 1.0, it is crucial to restrict the light transmitting layer thickness d to smaller values (e.g. <=3 μm) as it is easier to compensate for e.g. aberrations (Zijp et al., Proc. of SPIE Vol. 5380, p 209 ff.
In addition to the above mentioned optical properties of the HRI layer and due to the very small WD of NFR Optical Pick Up Heads, such HRI layer also should act as protection layer for the information stored in the recording medium and for the Optical Pick Up Head in case of an accidential head crash. Therefor the HRI layer should have a high scratch resistance and a low surface roughness Ra, as WD is in the range of only a few 10 nm. Also the absorption or imaginary part k of the index of refraction of the HRI layer should be low, in order to enable high enough reflection from multiple stacked recording layers, separated by spacer layers that may be comprised of the HRI layer and to realize high readout stability. Again, prior art did not have to account for such a complex property profile of the disc structure.